There is known a lock-up control as an earlier technology which directly connects an output shaft of a torque converter and an input shaft thereof at a predetermined engine operating condition to eliminate a transmission loss in an automatic transmission and improve a fuel economy. A Japanese Unexamined Patent Publication 3-185269 discloses a torque down control in a lock-up technology by which a jolt due to a difference in torque levels between the input shaft and the output shaft of the torque converter at a lock-up timing is reduced and a shift time of a lock-up is shortened.
This torque down control is performed by retarding ignition timing during a lock-up transition state from after a lock-up control start to before a lock-up completion.
However, the above technology raises the following problem because the ignition timing is retarded by a constant retard quantity during the lock-up transition state.
In an engine wherein the lock-up is performed not only at a fourth gear shift but also at a relatively low vehicle speed, low engine rotation speed, and high load region such as at a third gear shift, when the lock-up is performed at the third gear shift, decrease of an engine rotation speed and increase of an engine load due to the lock-up control change more rapidly, thereby bringing about combustion instability right after the lock-up and a temporal knocking thereafter.
On this occasion, in order to avoid the occurrence of the knocking immediately after the lock-up, it is necessary during the lock-up transition state immediately before the lock-up to, in advance, set retard of ignition timing at engine operating regions in an ignition timing table wherein the knocking occurs immediately after the lock-up and as a result, there is a problem that an engine torque is lowered at the engine operating conditions having the retarded ignition timing.